1. Field of the Invention
The present invention relates to a method and mold assembly for producing a molded object of plastic optical elements such as a lens, a mirror, and a prism.
2. Discussion of the Background
Metallic mold assemblies that employ an injection molding method to produce plastic optical elements such as plastic lenses with high accuracy are known. Known methods of producing a molded object 21 of a plastic optical element using a mold assembly 1 are shown in FIGS. 8A through 8C. As illustrated in FIG. 7, the molded object 21 has mirror surfaces 22 and 23 as optical surfaces, as well as non-optical surfaces 24 and 25. Referring to FIG. 8A, the mold assembly 1 includes a fixing-side metallic mold 2 providing a cavity piece 8 (i.e., a molding piece for forming a cavity) and a moving-side metallic mold 3 providing a cavity piece 9. The cavity pieces 8 and 9 have transfer surfaces 8a and 9a, respectively. A pair of porous members 26 is arranged opposing each other in the mold assembly 1. A cavity, which is filled with molten resin during molding, is defined by the cavity pieces 8 and 9 and the porous members 26. After the molten resin is loaded into the cavity, the molten resin is cooled under a controlled pressure. In the cooling process, when air is guided to the cavity via the porous members 26, the sides of the resin in the cavity corresponding to the non-optical surfaces 24 and 25 of the molded object 21 are pressed. This presses the resin in the cavity against the transfer surfaces 8a and 9a. As a result, the transfer surfaces 8a and 9a are transferred to the resin, and thereby the mirror surfaces 22 and 23 of the molded object 21 are formed.
In another mold assembly 1 illustrated in FIG. 8B, a vent hole 27 is provided at a side surface other than the transfer surfaces 8a and 9a in the cavity. Furthermore, a communication path 28 is provided to place the vent hole 27 in fluid communication with the exterior of the mold assembly 1. When the molten resin is loaded into the cavity, the communication path 28 is filled with compressed air in the cavity. Then, a differential pressure is generated between portions of the resin contacting the transfer surfaces 8a/9a and a portion of the resin contacting the vent hole 27. This forms a sink only at the portions of the resin that contact the vent hole 27. Thereby, a sink can be prevented from occurring at the mirror surfaces 22 and 23 of the molded object 21.
In still another background mold assembly 1 illustrated in FIG. 8C, a slide cavity piece 29 is provided to form a side surface of the cavity other than the transfer surfaces 8a and 9a. When the molten resin in the cavity is cooled to a temperature lower than a softening point of the resin, a gap 30 is forcibly formed between the resin and the slide cavity piece 29. This is done by sliding the slide cavity piece 29 in a direction away from the resin so as to form a sink only at a portion of the resin facing the gap 30. Thereby, a sink is prevented from occurring at the mirror surfaces 22 and 23 of the molded object 21.
In the above-described mold assembly 1, the molded object 21 of a highly accurate optical element can be obtained by forming a sink in a portion of the molded object 21 other than mirror surfaces 22 and 23 which are to be used as optical surfaces. This allows mirror surfaces 22 and 23 to be shaped by transfer surfaces 8a and 9a. This is further accomplished by reducing internal distortion of the molded object 21. However, because a portion of the resin where the sink is formed is separated from a piece of the mold assembly 1 at a temperature above the softening point of the resin, the thermal conductivity of the above-described portion of the resin where the sink is formed becomes extremely low. As a result, a significant cooling time is required to cool the resin.
Furthermore, when the sink is formed asymmetrically at a surface of the cavity other than transfer surfaces 8a and 9a as illustrated in FIGS. 8B and 8C, the temperature distribution of the resin becomes uneven. This is because the temperature of the portion of the resin where the sink is formed is relatively high and the temperature of other portions of the resin is relatively low. As a result, after the molded object 21 is removed from the mold assembly 1, the molded object 21 may deform due to differences in contraction rates. In order to prevent the deformation of the molded object 21, it is necessary to sufficiently cool the resin and to lower the temperature of the molded object 21 before removing the molded object 21 from the mold assembly 1. However, the process of cooling the resin requires a significant amount of time.
Furthermore, in the mold assembly 1 illustrated in FIG. 8C, when the slide cavity piece 29 is slid away from the resin, the molded object 21 may be deformed due to a change in the contact-force between the slide cavity piece 29 and the resin during the process of cooling the resin to a temperature lower than a softening point of the resin.